FIG. 1 illustrates an example GSM-based cellular wireless communications system 2. A plurality of mobile stations (MSs) 10 (sometimes called mobile terminals, user equipments (UEs), etc.) are located in a geographical service area covered by cells C1 through C6. Radio base stations (RBSs) 4 are positioned within the geographic area covered by the cells C1 through C6 and act as an interface between the mobile station 10 and the wireless communications system 2. The radio base stations 4 are typically connected to a base station controller (BSC) 6 or radio network controller (RNC), which in turn is connected to one or more core network nodes like a mobile switching center (MSC), a serving GSM support node (SGSN), etc. The BSC 6 may be connected to other BSCs, and the core network node(s) are usually coupled to external networks like a public switched telephone network (PSTN) 8 and/or the Internet.
The GSM wireless communications system 2 in FIG. 1 has only a limited frequency band it is allowed to use. Each cell is assigned a set of channels in the allowed frequency band. Each set of channels is reused after separation of a certain number of cells so that adjacent cells are assigned a different set of channels to reduce/alleviate co-channel interference. Thus, for wireless communications systems that divide the spectrum into narrow frequency bands like GSM, careful frequency planning is usually required. Each frequency band is divided into frames, and in GSM, each frame includes eight time slot radio channels. Thus, GSM is a time division multiple access (TDMA) system which divides a narrow frequency band into different time slots, where each repeating time slot in the frame can be assigned as a separate communications channel resource. The following steps transform speech audio to radio waves at a GSM transmitter and vice versa at a GSM receiver: speech coding, channel coding, interleaving, burst assembling, ciphering, modulation (e.g, GMSK modulation), and then radio transmission. If the source of information is data (not speech), the speech coding is not performed.
The rapid growth of the subscriber base in GSM has stressed the need for increased voice and data capacity. With the advent of machine-to-machine communications, GSM traffic is expected to grow even more. In addition, with the success of mobile broadband and smart phones, spectrum efficiency and hardware efficiency become even more important. Operators foresee the need to re-farm the GSM spectrum to WCDMA or LTE, but at the same time, GSM voice remains one of the main sources of revenue. All of these needs make it desirable to pack more GSM traffic into existing or fewer frequency bands. Mobile network operators and telecom equipment manufacturers have recently standardized the feature “Voice services over Adaptive Multi-user channels on One Slot” (VAMOS), which multiplexes two users onto one time slot channel instead of just one user. But despite the improvements brought about by VAMOS, it is important to optimize even further the hardware and spectral efficiencies of GSM and similar networks.